Smoke detectors

ABSTRACT

A smoke detector for use in detecting the early outbreak of a fire and including a housing defining a smoke chamber, a housing defining a reference chamber, a grid in each of the chambers and an electrostatic shield surrounding each of the grids. The shields are made of an electrically-conductive plastic material thereby eliminating the possibility of corrosion.

United States Patent [191 Van der Walt et al.

[11]. 3,824,422 1451 July 16,1974

SMOKE DETECTORS Inventors: Nicolaas T. Van der Walt;

Bernardus J. Bout; Timothy J. Newington, all of Johannesburg,, South Africa Assignee: Anglo American Corporation of South Africa Limited, Johannesburg, South Africa Filed: June 25, 1973 Appl. No.: 373,317

Foreign Application Priority Data June 27, 1972 South Africa 72/4440 Nov. 15, 1972 South Africa 72/8084 US. Cl 313/218, 313/54, 250/389 Int. Cl. H0lj 7/40 Field of Search 313/54, 218; 250/381, 389

References Cited UNITED STATES PATENTS 3,769,917 10/1973 Lampart et al. 313/54 Primary Examiner-Herman Karl Saalbach Assistant ExaminerDarwin R. Hostetter Attorney, Agent, or FirmY0ung & Thompson [5 7] ABSTRACT A smoke detector for use in detecting the early outbreak of a fire and including a housing defining a smoke chamber, a housing defining a reference chamber, a grid ,in each of the chambers and an electrostatic shield surrounding each of the grids. The shields are made of an electrically-conductive plastic material thereby eliminating the possibility of corrosion.

4 Claims, 1 Drawing Figure SMOKE DETECTORS This invention relates to smoke detectors. Such detectors are used in many applications but are becoming increasingly important in mines for providing an early warning of the outbreak of a fire.

The conventional smoke detector usually consists of two parts: a first part known as the smoke chamber and a second part known as the reference chamber. A grid is located in each chamber. These detectors operate on very small currents and forthis reason the chambers are generally provided with one or more electrostatic shields surrounding the grids. The shields are normally made of metal and they tend to corrode in the presence of corrosive fumes in the mines. Also essential to the operation of these detectors are two or more electrodes two of which must be exposed to the atmosphere. These electrodes also tend to corrode due to corrosive fumes.

It is an object of the present invention to provide an improved smoke detector in which the abovementioned problem of corrosion is eliminated.

In a smoke detector according to the invention the electrostatic shields are made of an electricallyconductive plastic material.

Further accordingto the invention the smoke chamber housing itself constitutes the electrostatic shield for the grid in the smoke chamber and'the smoke chamber housing is made of an electrically-conductive plastic material.

Further according to the invention the reference chamber housing itself constitutes the electrostatic shield for the grid in the reference chamber, and the reference chamber housing is made of an electricallyconductive material.

Further according to the invention, the grids are made of an electrically-conductive plastic material.

To illustratethe invention an embodiment thereof is described hereunder with reference to the accompanying drawing which is a sectional side elevation of the smoke detector.

With reference to the drawing. a cylindrical body is closed at one end by an end plate 12 and at the other end by a removable cover plate 14. The cover plate is secured to the body by means of screws 16.

A cylindrical chamber 18 is defined by two cupshaped sections 20, 22 and is mounted inside the body 10. The chamber 18 constitutes the so-called reference chamber and cannot easily be penetrated by smoke particles. i

A printed circuit board 24 is mounted on the upper end of the reference chamber 18 by means of screws 26. The printed circuit board is attached to the body 10 by screws 28 and has mounted on it the electronics 30 normally associated with detectors of this type.

An electrode or grid 32 is located in the reference chamber 18.

A cup-shaped housing 34 is attached to the lower end of the reference chamber 18 and defines a chamber 36. The chamber 36 is the so-called smoke chamber.

A second electrode or grid 38 is located in the smoke chamber 36. The two grids 32 and 38 are electrically inter-connected by means of conductors 40 one of which leads to the electronics 30.

A radio-active source 42 extends into the smoke and reference chambers 36 and 18.

In operation, ions generated by the radio-active source allow small ionization currents to flow between the grid 32 and the cup-shaped sections 20 and 22 defining the reference chamber 18, and between the grid 38 and the cup-shaped housing 34 defining the smoke chamber 36. The sections 20 and 22, and the housing 34 therefore act as electrodes.

The two ionization paths are connected in adjacent paths of a bridge circuit in the electronics 30.

The presence of smoke particles in the smoke chamber will cause a reduction of the ionization current between the grid 38 and the housing 34 and thus unbalance the bridge. The electronics 30 detects'the unbalance and energizes a suitable alarm.

No novelty is claimed in respect of the detector describedabove and the description is given merely for the purpose of understanding the invention.

It is normal, in detectors of thist'ype, to provide electrostatic shields around the grids 32 and 38. As indicated however, the sections 20, 22 and the housing 34 act as electrodes during operation of the detector and they can therefore conveniently be employed as electrostatic shields as well. g l

In the present invention the sections 20, 22 and the housing 34are made of an electrically-conductive plastic material. In such a material, selected'quantities of carbon black or metal dopes are mixed with plastic to impart a resistivity to the material of the order of 10' ohms per cubic centimetre.

Detectors of the type described here are high impedance devices and operate on very small currents. The conductive plastic mentioned above is thus adequate as a conductor at these small currents. More importantly however, the problem of corrosion of the sections 20, 22 and the housing 34 is completely eliminated.

In an alternative form of the invention the grids 32 and 38 are also made of conductive plastic.

We claim:

l. A smoke detector of the type including a housing defining a smoke chamber, a housing defining a reference chamber, a grid in each of the chambers and an electrostatic shield surrounding each of the grids, characterised in that the electrostatic shields are made of an electrically-conductive plastic material.

2. A smoke detector as claimed in claim 1 characterised in that the smoke chamber housing itself constitutes the electrostatic shield for the grid in the smoke chamber and in that the smoke chamber housing is made of an electrically-conductive plastic material.

3. A smoke detector as claimed in claim 1 characterised in that the reference chamber housing itself constitutes the electrostatic shield for the grid in the reference chamber and in that the reference chamber housing is made of an electrically-conductive plastic material.

4. A smoke detector as claimed in claim 1 characterconductive plastic material. 

1. A smoke detector of the type including a housing defining a smoke chamber, a housing defining a reference chamber, a grid in each of the chambers and an electrostatic shield surrounding each of the grids, characterised in that the electrostatic shields are made of an electrically-conductive plastic material.
 2. A smoke detector as claimed in claim 1 characterised in that the smoke chamber housing itself constitutes the electrostatic shield for the grid in the smoke chamber and in that the smoke chamber housing is made of an electrically-conductive plastic material.
 3. A smoke detector as claimed in claim 1 characterised in that the reference chamber housing itself constitutes the electrostatic shield for the grid in the reference chamber and in that the reference chamber housing is made of an electrically-conductive plastic material.
 4. A smoke detector as claimed in claim 1 characterised in that the grids are made of an electrically-conductive plastic material. 